How to Avoid Moisture Build-up in the Roof Cavity

Energy efficiency, humidity, and airflow are in the spotlight with changes to H1 Energy Efficiency in the New Zealand Building Code. It’s important to highlight the effect these factors have on the roof cavity and condensation build up within roofing systems. The use of modern materials such as long run metal roofing, population densification, and a trend towards more airtight building envelopes has the potential for increased moisture levels in roof cavities and issues for the homeowner. But with considered design and materials choices, these issues can be avoided.

NZ’s lack of clarity on roof ventilation requirements

We start by looking at the New Zealand Building Code, sometimes referred to as the guideline to create ‘the worst performing building you can legally build’. A closer look at the ventilation requirements in roofs highlights this clearly. No measurements for airflow, no clarity on creating airflow pathways, no guidelines on ‘what good looks like’. In comparison, countries like Australia have clear mandates on how much airflow at the eaves versus the ridge is required within their building code.

Alongside this lack of clarity, New Zealand faces rising complexity within building design as MBIE drives rapid changes in acceptable building products and new pathways to compliance. Even the modernisation of building materials has had an effect — something as simple as the recent change in underlay from bituminous to synthetic has had a wide-reaching effect when it comes to condensation and mould.

How intensification has increased moisture

In any home a volume of moisture will travel upwards through the ceiling and, in cold roof designs, into the roof cavity. This is exacerbated by the densification of our cities. Each additional person per dwelling adds moisture into the building through showers, cooking, laundry, and even breathing. A family of four can produce up to seven litres of moisture per day just from breathing.

Design trends towards lower pitched roofs make the problem worse with an increased volume of moisture accumulating due to reduced airflow. The air escaping upwards needs an exit as well as a force to draw that moisture-laden air towards it. Ventilation gaps in the underlay and at the ridge create an exit route for this air, and adequate ventilation at the eaves creates that upwards force.

How material choice impacts moisture accumulation

Another consideration is the roofing material itself and how it reacts to these changes in temperature.

The shift away from traditional concrete tile roofing

Traditional concrete tile roofing is naturally breathable, with gaps between each tile allowing for airflow and weepholes at ridges for escaping air. However, NZ is trending away from using concrete tile, or its similar cousins — slate and terracotta. Generally seen as too much of a risk in earthquake prone areas, the heavy material is difficult to transport, requires more structural framing, and is prone to cracking.

Moisture issues with metal sheet roofing

Metal sheet roofing, also variously known as long run, tray, or profiled metal roofing is a common roofing material. But this material creates new issues with condensation. The roof buildup shows the purlins installed with underlay over the top. The roofing is then placed directly on top of the underlay creating touch points where the trough rests on the underlay. When the metal roof cools down overnight, these touch points create cold spots on the underlay. At these cold spots moisture within the roof cavity cools down and can condense on the underside of the underlay. When critical mass is reached droplets fall into the roof cavity creating damp spots, mould, and rot. As a solution, some designers are introducing ventilated battens above the underlay to allow for separation, adding more cost and complexity within the build.

Metal tile roofing — a build-up designed to avoid moisture

Metal tile roofing is a hybrid of both systems, with each tile having gaps between them allowing the roof to breathe. It’s laid in a similar fashion to concrete tile, but with the lighter structure of a metal sheet roof. The roof buildup keeps the underlay away from the metal tile by installing a batten in between the two. This keeps moisture in between the underlay and the roof tiles, allowing morning sun to evaporate and expel the moisture without it escaping into the roof cavity.

The extent of the breathability of metal roof tiles was tested by Gerard, and found that per standard Shake tile, there was an estimated free vent area of 2000mm². This demonstrates that without additional ventilation throughout the body of the roof, metal tile roofing has a natural channel for expelling air, making it an excellent choice for a drier roof.

While the way we calculate energy efficiency may change, the underlying principles behind what’s happening in our homes hasn’t. Considered design and materials choice can avoid condensation issues and lead to healthier home outcomes.